T.D.
First of all, to get a good nights sleep, I feed my 3 month old until he just can't eat anymore starting about 7pm. By 8 or 8:30 he is fast asleep and generally sleeps until 5 or 6 am. I do supplement breast feeding with formula and this is the time I give the formula. If you don't, consider giving a bottle of breast milk as the baby can drink more/faster from the bottle and you can get them to keep drinking even if they fall asleep. (You can also try a "Dream Feed", later in the evening, feed without waking - it's a challenge but can be done.) If the baby can get a good nights sleep you'll have a clean start the next day.
I like the idea of the sling others have suggested, my son will sleep very well in the sling, but that's not always practical for me. At those times, I use his swing or put on a video to occupy the 2 year old so that I can hold the baby to put him to sleep.
I have done my best to teach my 2 year old that there are times that I need to hold the baby and there are times I can devote completely to her. My days go by much better when I make a point to give the older one her special time, she tends to allow her brother his due more willingly. I also try to incorporate her "help" with the baby, making her feel useful and more involved and less ignored.
Best wishes to you - I know it's a challenge.